1. Field of the Invention
This invention relates to axial flow rotary machines, and more particularly to an air seal between the tips of the machine airfoils and circumscribing portions of the flow path wall.
2. Description of the Prior Art
The concepts of the present invention are described with respect to a compressor embodiment thereof in a gas turbine engine. In such a compressor a plurality of rows of rotor blades extend radially outward from a rotor shaft across a flow path for the working medium gases. Collaterally, a plurality of rows of stator vanes extend radially inward across the flow path from a stator case. In some embodiments the stator vanes are cantilevered inwardly from the stator case. Each row of stator vanes is positioned to direct the medium gases into or away from an adjacent row of rotor blades. A stator seal land extends from the stator case to circumscribe the tips of the blades of each blade row. In cantilevered vane embodiments a rotor seal land extends from the rotor to circumscribe the tips of the vanes of each vane row.
The aerodynamic efficiency of the compressor is largely dependent upon the clearance between the tips of each row and the corresponding seal land. As the clearance is increased, substantial amounts of working medium gases leak circumferentially over the tips of the airfoils from the pressure sides to the suction sides of the airfoils. Additionally, amounts of medium gases leak axially over the tips from the downstream end to the upstream end of the airfoils.
The historic approach in controlling leakage has been to minimize the clearance dimension between the tips and the corresponding seal land at the design operating condition. Such, however, is not an easy task as during operation of the machine the relative radial growths between the tips and the corresponding seal lands are substantial. For example, as the rotor is turned to speed, thermal expansion of the rotor materials and centrifugally generated forces cause the tips of the rotor blades to be displaced radially outward toward the corresponding stator seal land. Sufficient initial clearance between the tips and the seal land must be provided to prevent destructive interference during this initial period. As thermal equilibrium is reached the stator seal land grows radially away from the blade tips to produce a resultant and undesirable clearance gap. Corresponding effects occur in cantilevered stator designs.
In an effort to avoid unduly large initial clearances many modern engines utilize abradable seal lands in which the airfoil tips are allowed to wear into the lands during transient excursions. U.S. Pat. Nos. 3,519,282 to Davis entitled "Abradable Material Seal"; 3,817,719 to Schilke et al entitled "High Temperature Abradable Material and Method of Preparing the Same"; 3,843,278 to Torell entitled "Abradable Seal Construction"; and 3,918,925 entitled "Abradable Seal" are representative of such seals and their methods of manufacture. Accordingly, by such embodiments the clearance over the airfoil tips becomes the minimum clearance that will accommodate rotor excursions.
Other techniques for reducing leakage across the blade tips have been investigated. One such technique relevant to the presently disclosed concepts is reported in NASA Technical Memorandum X-472 by Kofskey entitled "Experimental Investigation of Three Tip-Clearance Configurations Over a Range of Tip Clearance Using a Single-Stage Turbine of High Hub to Tip-Radius Ratio". Specifically, the "recessed casing" reported in the memorandum and illustrated in FIG. 3(b) is of interest. In accordance with the Kofskey teaching improved efficiency over conventional, smooth wall seals is obtainable by submerging the tips of turbine blades into a recess in the corresponding seal land. A comparison of smooth wall and recessed casing efficiencies is shown in FIG. 8 of Kofskey. Also shown in Kofskey is a comparison in FIG. 6 between a recessed casing in which the blade tips are submerged and a recessed casing in which the blade tips run line on line with the flow path wall. The tests show the submerged construction to be markedly superior by several percentage points in efficiency.
As energy costs continue to soar, manufacturers of rotary machines are devoting substantial resources to the improvement of machine efficiencies. It is against this background that the present inventive concepts were developed.